This application claims the priority of German Patent Application Serial No. 199 56 954.1, filed Nov. 16, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to a parting tool for continuous chipless separation of identical parts from a rotation-symmetric workpiece.
International PCT patent application WO 95/29777 describes a parting tool for the continuous chipless separation of single identical rings from tubular workpieces. The parting tool includes three cutting rollers which are driven in a same direction and arranged around the tubular workpiece being worked on. The cutting rollers are interconnected through positive engagement, with each cutting roller including a cylindrical main body which has axial ends defining a slip bevel and a calibrating portion, respectively, and a screw thread with thread-shaped cutting edges disposed between the slip bevel and the calibrating portion. The cutting edges include, as viewed in longitudinal section, a pointed cutting head and a cutting foot with flanks that are oriented substantially perpendicular to the respective roller axis. The flanks have a width which increases from entry to exit. The contour in the bottom land between the cutting edges of the cutting roller is substantially parallel to the roller axis, and the diameter of each cutting roller increases from beginning toward the end.
This conventional parting tool suffers shortcomings because it fails to take into account the radial material flow so that the fabrication of blanks from the workpiece with precise shape (straight, flat end faces and accurate cylindrical surface area) cannot be realized or only realized at great expenditure. As the ridge formation cannot be influenced in a desired manner, the material loss during subsequent mechanical processing of the blank is substantial. Calibration, i.e. implementation of a precise end diameter of the blank, can be realized only to a limited extent and is dependent on the wear of the cutting rollers and on the roller adjustment. This conventional parting tool is also incapable to fabricate blanks with an outer diameter that is greater or smaller than the outer diameter of the initial rotation-symmetrical workpiece. Moreover, this conventional parting tool is subject to high wear so the its service life is inadequate.
A conventional parting tool of a type involved here is shown, for example, in FIG. 1. The parting tool includes a cutting roller 10 (only a section thereof is shown here for ease of illustration) having a screw thread 12 with coils, whereby each coil has a cutting head 13 of pointed configuration. This is disadvantageous as indicated by the cross section of the produced blank 11 because during cutting operation, material migrates into the space created by the pointed configuration of the screw thread 12 and results in the formation of lobed ridges 14, 14xe2x80x2 on the produced blank 11, resembling the shape of half of a hysteresis loop. This leads to high material loss during subsequent mechanical treatment and longer processing times. Moreover, the feed of blanks 11 into the processing machine is more difficult as the blanks 11 may interlock as a consequence of the pointed ridge configuration.
In conventional parting tools, the cutting roller is so configured that the calibrating portion forms an integral part of the cutting roller. This is, however, disadvantageous because a wear of this portion requires a replacement of the entire cutting roller.
It is thus an object of the present invention to provide an improved parting tool, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved parting tool which is less susceptible to wear and thus exhibits a long service life and yet is capable to produce blanks of high quality with precise shape and to keep a material loss to a minimum.
It is still another object of the present invention, to provide an improved parting tool which is capable to produce blanks with an outer diameter that can be greater or smaller than the outer diameter of the processed rotation-symmetrical workpiece.
According to one aspect of the present invention, a parting tool includes a cutting roller having a main body advancing a workpiece in a transport direction from an entry side to an exit side, with the main body having a slip bevel at the entry zone and a calibrating portion at the exit zone and a cutting screw disposed between the slip bevel and the calibrating portion and including a screw thread having coils which define a helix line and are configured with an increase in height, whereby the screw thread includes in series an entry portion, a substantially cylindrical primary portion, a separating portion and the calibrating portion, with the coils in the entry portion, primary portion, separating portion and calibrating portion having different slope angle, with the slope angle determined by a combination of a thickness of the screw thread and a root width between two neighboring coils of the screw thread, wherein the increase in height of the coils correlates to the helix line.
A parting tool in accordance with the present invention is able to consider the radial material flow and to attain an optimal blank shape as the space between neighboring coils of the screw thread up to the bottom land of the main body is sufficiently filled. At the same time, pressure on the flanks of the coils is reduced as a same pressure is applied on both sides of the flanks so that the screw thread is exposed to reduced wear and the service life of the cutting roller is extended.
The individual parts of the cutting roller assume the following functions:
a) Slip Bevel
Centers and guides the entering rotation-symmetrical workpiece.
b) Entry Portion
Commencement of the cutting thread portion, with the first coils intended to grab the workpiece and to apply a tension force on the workpiece. Preferably, the root diameter in this portion slightly increases, i.e. it is slightly conical.
c) Primary Portion
In this portion, the height of the coils significantly increases. This portion represents the start of the actual separation process, with formation of the ridge at the end face. This portion is long enough to just about effectuate a cutting of the workpiece.
d) Separating Portion
Typically, this portion is provided with one or more coils to ensure that even when the coils wear off, resulting in a decrease of the coil height, a secure cutting operation of the workpiece is accomplished. This portion results in a final formation of the end faces.
e) Calibrating Portion
In this portion, the final outer diameter of the blank is established and the exact cylindrical shape is ensured. Typically, this portion is devoid of any coils, although in special cases coils may be provided. The root diameter of this portion is slightly greater than the root diameter of the preceding separating portion. A kind of xe2x80x9cpre-calibrationxe2x80x9d may also be established in the preceding separating portion.
In the description, the term xe2x80x9croot diameterxe2x80x9d in the respective portion will denote the diameter of the main body, bounding the root or base of the screw thread.
According to another feature of the present invention, the screw thread defines an ascending line which is tangent to the crests of the coils in the entry portion and a transition to the primary portion, whereby the ascending line extends substantially perpendicular to the helix line. This positively influences the size of the created ridge and the wear behavior of the screw thread.
The material displacement may also be optimized when suiting the root diameter in the entry portion, the primary portion, the separating portion and the calibrating portion to the cutting roller at hand. Suitably, the root diameter in the individual portions remains constant, whereas the main body of the cutting roller steadily increases in transport direction.
According to another feature of the present invention, the bottom land defined between neighboring coils of the screw thread may be slightly conical or asymmetrically concave, with the ascending section extending in transport direction of the cutting roller.
The type of ridge formation is especially impacted by the configuration of the top land of the coils. Suitably, the coils of the screw thread in the entry portion, primary portion, separating portion and calibrating portion have different top lands. For example, the top land in the entry portion and the top land in the primary portion may be pointed at an acute angle of  less than 90 degrees, and the top land in the separating portion and the top land in the calibrating portion may be flat. Another variation includes the configuration of the top land of the coils in the entry portion in the form of a double-pointed tip.
Depending on the application at hand, it may be suitable to configure the top land in the entry portion and the top land in the primary portion with one-sided pointed tip, whereby the tips of some coils face the entry zone and the tips of some other coils face away from the entry zone, with the change in orientation occurring after every 360xc2x0 with a transition zone.
According to another aspect of the present invention, a parting tool assembly includes a pair of cutting rollers disposed above one another at formation of a gap for passage of a workpiece in a transport direction, each said cutting roller including a main body having axial ends defining an entry zone and an exit zone, respectively, with the main body having a slip bevel at the entry zone and a cutting screw disposed in succession of the slip bevel and including a screw thread having coils which define a helix line and are configured with an increase in height, with the screw thread including in series an entry portion, a substantially cylindrical primary portion in succession of the entry portion, and a separating portion disposed at the exit zone, wherein the entry portion, primary portion and separating portion have different slope angle, with the slope angle determined by a combination of a thickness of the screw thread and a root width between two neighboring coils of the screw thread, wherein the increase in height of the coils correlates to the helix line; bearing members for supporting the axial ends of the main body, and at least two exchangeable calibrating rollers disposed in transport direction behind the bearing member at the exit zone of the cutting rollers.
In this manner, the calibrating portion is divorced from the cutting roller so that the separating portion forms the end of the cutting roller. The main body of each cutting roller is supported on a shaft, whereby one of the calibrating rollers may be positively connected with the shaft of one of the cutting rollers and another one of the calibrating rollers may be resiliently and loosely supported on the shaft of the other one of the cutting rollers. As a result, it is possible to use pressure-applying rollers to so adjust the axial disposition of the resilient calibrating roller that the blank exits the parting device at a smaller dimensional tolerance of the outer diameter.
Suitably, the diameter of the calibrating roller is greater than the root diameter of the separating portion of the cutting roller by at least 2%, and the width of the calibrating roller is greater than a width of each produced blank cut from the workpiece. Preferably, the width of the calibrating roller is twice as wide than the width of each of the produced blanks. Whether separate calibrating rollers or integrated calibrating portion are involved, same considerations apply for realizing an optimum width.
To further minimize wear, the cutting roller may be made of multiple parts, whereby the single portions are made of different materials. Suitably, the slip bevel and the entry portion are made of a material of high wear resistance, and the primary portion and the separating portion are made of conventional hot-working steel. An example for such hot-working steel includes a grade of steel with the designation X 32 CrMoV 3.3 with a hardness in the range of 48 to 51 HRC. An example for a material with increased wear resistance includes a special grade of steel with the designation thyrotherm 2999 EFS Supra with a hardness in the range of 54 to 57 HRC, a high resistance to temperature fluctuations and a high resistance to wear as a result of heat.
Of course, the cutting roller may also be configured of single-piece construction, whereby different strength and toughness may be implemented through application of particular coats. The main body may be of uniform construction, with the entry portion, primary portion and separating portion being applied on the main body as layers of different materials. For example, the slip bevel and the entry portion may include a thick layer applied through surfacing by welding a powder of a hard material alloy with high wear resistance. It may also be possible to apply on the entry portion and/or, optionally, on the calibrating portion, a thin layer made of a hard material alloy, e.g. TiAlN, applied by a sputtering process.
According to another feature of the present invention, the various portions of each cutting roller may have different surface structure to attain further improvements. For example, the calibrating portion may have a polished surface structure, whereas the other portions may have a milled surface structure. Also, the selected surface structure may be combined with a suitable type of coating.
All described measures result individually or in combination in an improvement of the radial material flow and to a reduction in wear and thus to an increase of the service life of the cutting rollers. Moreover, the embodiment of a parting tool with separate calibration permits the production of blanks that have an outer diameter which is greater or smaller than the diameter of the initial workpiece.